<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
 <button id="btn1">按钮1</button>
 <button id="btn2">按钮2</button>
 <button onclick="demo()">按钮3</button>

  <script type="text/javascript">
    const  btn1 = document.getElementById('btn1');
    const  btn2 = document.getElementById('btn2');
    btn1.addEventListener('click',()=>{
      alert('按钮1被点击了')
    })
    btn2.onclick = ()=>{
      alert('按钮2被点击了')
    }
    function demo(){
      alert('按钮3被点击了')
    }
  </script>
</body>
</html>
